An optical fiber sensor is a system which consists mostly of an optical fiber and has a detecting element provided at some point in the optical path of the optical fiber. The detecting element is a component that undergoes a change in the characteristic thereof in accordance with a quantity of an object to be detected. For example, in case a single mode fiber is used as the detecting element for sensing an external disturbance such as vibration, pressure, temperature, electric field, magnetic field or acoustic vibration, the disturbance is detected by a fiber interferometer in the form of a change in the optical path of the single mode fiber caused by the external disturbance.
With such an optical fiber sensor, however, such a problem may occur as fluctuation of the output interference fringe or disappearance of signal caused by an accidental change in the state of polarization of light due to birefringence taking place in the optical fiber.
To address this problem, Electronics Letter 14; Mar. 1991 Vol. 27, No. 6 proposes to use a Faraday rotator mirror in part of the fiber interferometer. The Faraday rotator mirror is an optical component that suppresses variations in the state of polarization caused by birefringence in the optical fiber and maintains the state of polarization of the input light.
FIG. 20 is a sectional view schematically showing the constitution of a Faraday rotator mirror 321 of the prior art. The Faraday rotator mirror 321 comprises an optical fiber 322, a coupling lens 323, a Faraday rotator 325, a reflector mirror 326 and a magnet 327.
The optical fiber 322 is a single mode fiber. The coupling lens 323 is a member used to efficiently couple light reflected on a reflector mirror to be described later to the optical fiber 322, and is disposed so as to oppose one end of the optical fiber 322. The Faraday rotator 325 has the function to give rotation of a predetermined angle to the state of polarization of the incident light by applying a predetermined magnetic field, and is formed from, for example, bismuth-substituted garnet crystal. The Faraday rotator 325 is formed with such a thickness, for example, that causes rotation of 45° in the state of polarization of the incident light. The reflector mirror 326 is a member used to reflect light emerging from the optical fiber 322, and is disposed so as to oppose one end of the optical fiber 322 via the coupling lens 323 and the Faraday rotator 325. The magnet 327 is used to apply a magnetic field of predetermined intensity (for example, magnetic field of saturation of bismuth-substituted garnet crystal or higher) to the Faraday rotator 325.
FIG. 21 is a diagram explanatory of the state of polarization of light in the Faraday rotator mirror 321 viewed from the optical fiber 322. Principle of operation of the Faraday rotator mirror 321 will be described below by making reference to FIG. 21. For the sake of convenience, light emerging from the optical fiber 322 will be called the incident light, light reflected on the reflector mirror 326 will be called the reflected light, propagating direction of the incident light will be called the forward direction and propagating direction of the reflected light will be called the reverse direction. Although the state of polarization of the incident light is assumed to be linear polarization, this does not restrict the present invention which can be applied to a case of any state of polarization.
First, incident light (the symbol a in FIG. 21) emerging from the optical fiber 322 undergoes rotation of state of polarization by 45° clockwise viewed in the forward direction (the symbol b in FIG. 21) while passing through the Faraday rotator 325. The light reflected on the reflector mirror 326 (the symbol c in FIG. 21) reenters the Faraday rotator 325 in reverse direction. The reflected light undergoes rotation of the state of polarization by 45° clockwise viewed in the forward direction (the symbol d in FIG. 21) while passing through the Faraday rotator 325 in reverse direction, and enters the optical fiber 322. As a result, the light reflected by the Faraday rotator 325 has polarization perpendicular to that of the incident light, and has undergone birefringence in the opposite sense to that received during forward propagation so that the output is stabilized in the state of polarization orthogonal to the state of polarization of the input.
The Faraday rotator mirror 321 as shown in FIG. 20 has been applied to, in addition to the optical fiber sensor system, optical fiber amplifier system. The optical fiber amplifier system commonly uses an erbium-doped single mode fiber (several tens to several hundreds of meters long), and therefore suffers such problems that the state of polarization undergoes variation due to birefringence taking place in the optical fiber and the divergence of polarization mode which deteriorates the signal waveform in a long distance fiber-optic communications system. However, use of the Faraday rotator mirror 321 compensates these deviations thereby achieving stable output.
Japanese Patent No. 3,548,283 also describes the use of an enlarged-core fiber (having the same outer diameter as the optical fiber) in place of the coupling lens in order to make the system smaller. For the purpose of reducing the number of manufacturing processes and simplifying the assembly process, it has also been proposed to form a reflecting film on one end face of the Faraday rotator, or put the enlarged-core fiber and the Faraday rotator into contact with each other via an optically compatible adhesive.
FIG. 22A is a sectional view showing the constitution of the Faraday rotator mirror 331 described in Japanese Patent No. 3,548,283. The Faraday rotator mirror 331 is composed of an enlarged-core fiber 333, a Faraday rotator 335, a reflecting film 336, a cylindrical magnet 337 and an optically compatible adhesive 338.
The enlarged-core fiber 333 comprises a core 333a and a cladding 333b, as shown in FIG. 22B. The enlarged-core fiber 333 is manufactured by applying local heating to an ordinary single mode fiber. In the heating process, Ge and other dopant in the core 333a are thermally diffused so as enlarge the core 333a. The Faraday rotator 335 is a component having a constitution similar to the Faraday rotator 325. The reflector film 336 is formed from multi-layer dielectric material directly on one end face of the Faraday rotator 335. The reflecting film 336 has high reflectivity (such as 99% or higher) with low loss in light intensity. The cylindrical magnet 337 is a component having a function similar to that of the magnet 327 described previously.
In general, angle of divergence of a light beam emerging from an optical fiber becomes smaller and approaches collimated light, as the core diameter becomes larger. A larger angle of divergence makes it difficult for the reflected light to couple into the enlarged-core fiber 333. The constitution described in Japanese Patent No. 3,548,283 suppresses the efficiency of coupling from decreasing, in other words suppresses the insertion loss from increasing, by enlarging the core diameter three to four times. On the other hand, divergence of the beam increases as the distance between the end of the enlarged-core fiber 333 and the reflecting film 336 increases, thus resulting in lower efficiency of coupling. For this reason, the Faraday rotator 335 is installed in close contact with the enlarged-core fiber 333. In Japanese Patent No. 3,548,283, the optically compatible adhesive 338 has an extremely small thickness of 10 μm or less.
In Japanese Patent No. 3,602,891, it is proposed to suppress undesired reflected light from reentering the system by using an enlarged-core fiber and a Faraday rotator having trapezoidal shape (FIG. 23).
The Faraday rotator mirror 341 disclosed in Japanese Patent No. 3,602,891 is composed of an enlarged-core fiber 343, a Faraday rotator 345, a reflecting film 346, a cylindrical magnet 347 and an optically compatible adhesive 348.
The enlarged-core fiber 343 differs from that of the Faraday rotator mirror of Japanese Patent No. 3,548,283, in that one end face thereof (the face opposing the Faraday rotator 345) is tilted with respect to a plane perpendicular to the optical axis. In the Faraday rotator mirror 341 disclosed in Japanese Patent No. 3,602,891, the Faraday rotator 345 is connected on one end thereof with the enlarged-core fiber 343, while the other end face is disposed perpendicular to the optical axis (trapezoidal shape in the case of FIG. 23).